This invention relates in general to target tracking systems wherein radiation transmitted from a source to the target through a propagation medium is reflected from the target and detected at an echo receiving location to provide target range and imaging information.
The limiting distance for underwater target ranging systems, was heretofore determined from the nature of the target echo, the strength of the target illuminating radiation and the condition of the water as the propagating medium affecting signal backscatter and attenuation. For optical radiation the attenuation is quite large in water by virtue of which acoustic radiation is preferred. On the other hand, image resolution achieved by optical systems is much better than acoustic systems. Hence, an optical system is preferred for short range operation whereas an acoustic system is preferred at times when the turbidity of the water is so great as to render optical radiation useless because of its inability to penetrate the medium a sufficient distance to reach expected targets. It is well known in this regard that acoustic signals of very high frequency (hundreds of kilohertz) are not attenuated in water as rapidly as optical signals and are relatively immune to optical turbidity created by particulate matter in the water. However, acoustic pulse signals at very high frequencies are only useful for ranging and imaging purposes over limited distances and are not as good as optical radiation in regard to image resolution because of longer wavelengths.
The concept of providing dual modes of transmitting target tracking radiation is already known, as disclosed for example in U.S. Pat. No. 3,978,342 to Hagen et al. However, the alternative operational modes disclosed in the Hagen et al. patent involve in common the transmission of optical radiation. The use of acoustical means in connection with the transmission of targeting radiation from an optical source is also known, as disclosed for example In U.S. Pat. No. 4,690,551 to Edwards et al. The acoustic means according to the Edwards et al. patent is however used to modify the radiation transmitted, which remains optical in nature. The use of both optical and acoustical means for target sensing operations in a radiation tracking system is also disclosed in U.S. Pat. No. 4,761,072 to Pryor. Use of the acoustical means is however limited to the data readout phase of operation according to the Pryor patent. It is therefore an important object of the present invention to provide a target tracking system directly benefiting from the different characteristics of optical and acoustical radiation in producing a wider range of useful data obtained by emission of target illuminating radiation and reception of echo radiation reflected from the illuminated target.